1. Field of the Invention
The present invention refers to a pair of hard material plates intended to form the control members for a mixing valve of the so-called sequential type.
2. Description of the Prior Art
The so-called "sequential" mixing valves are those in which a single control member is movable with a sole degree of freedom and, during a complete control stroke starting from a rest position in which the delivery may possibly be shut off, during a first stage allow delivering only cold water, whose delivery rate increases up to a pre-established maximum, then during a second stage (stage of mixing) they effect the mixing of hot water in an increasing ratio with respect to the cold water, up to delivering only hot water, and finally in a third stage they reduce the delivery rate of the hot water, in theory up to attaining again a shut off condition, which however, in the practice, in most cases is hindered for reasons of safety, in order that then the valve could not be inadvertently opened in a condition of delivery of hot water only, which could be dangerous for the user.
In many known embodiments, the extension of the mixing field allowed by the valve is very limited, mainly in those cases in which the delivery takes place through a means opposing a noticeable resistance to the water flow, such as a shower.
In the known embodiments, the mixing stage is done in general by gradually increasing the passage section offered to the hot water and at the same time reducing the passage section offered to the cold water. By this provision it is aimed to maintain substantially constant the delivery rate of the mixed water, but the law of change of the temperature of the delivered water thus obtained is not very satisfactory.
In a mixing device already developed by the same Inventor (Italian Patent No. 722,831), which however refers to a non-sequential valve, a mixing process has been proposed in which the mixing stage is divided in two portions: during the first portion of the mixing stage the passage section offered to the cold water remains constant whilst the passage section offered to the hot water increases, whereas during the second portion of the mixing stage the passage section offered to the hot water remains constant whilst the passage section offered to the cold water decreases. As a result, a certain change in the delivery rate of the mixed water takes place, but the law of change of the temperature of the delivered water thus obtained is more satisfactory. However this known device uses a shutter in the shape of a tap member movable with two degrees of freedom, which could not be employed in a sequential mixer valve nor in a valve using hard material plates.
Moreover, in the known embodiments of sequential mixing valves using hard material plates, the one plate being fixed and the other movable, at least the movable plate has elongated passage openings which cooperate with corresponding passage openings of the fixed plate. These shapings of the passage openings require a relatively large diameter of the plates in order to allow a sufficient delivery rate, and therefore give rise to valves, or cartridges for valves, whose encumbrance is relatively large. In addition, there is an extensive contact surface between the fixed and the movable plates, and therefrom a relatively high friction ensues, and a resulting difficulty in the maneuver of the valve. Finally, the manufacture of the plates is rendered costly by the fact that different molds are required for manufacturing the fixed and the movable plates, which need to be different from each other.
The cited disadvantages have different importance in different applications, whereby pair of plates are manufactured in which both plates have very different shapes, which are intended to satisfy at least in part to different requirements. A change in the shape of the movable plate usually involves the need of changing the shape of the fixed plate too. This leads to considerable costs for manufacturing a number of plates having different shapes.